Energy saving is desired in view of the importance of preventing the global warming, generating a demand for a thermal insulation material having good thermal insulation properties as one means for energy saving.
A vacuum thermal insulation material, which has good thermal insulation properties among thermal insulation materials, has a wide range of application and is therefore required to exhibit further improved properties.
The vacuum thermal insulation material exhibits good thermal insulation properties by eliminating as much gas that transmits heat as possible so that heat transfer through gas is reduced. In order to improve the thermal insulation properties of the vacuum thermal insulation material, it is necessary to reduce the internal pressure of the vacuum thermal insulation material to suppress the heat transfer through gas which is due to collision of molecules.
However, the practically reachable degree of vacuum at the industrial level is approximately 10 Pa, and gas generated inside the vacuum thermal insulation material and an invasive gas component that enters the vacuum thermal insulation material from outside by permeation with time, for example, also cause temporal degradation of the thermal insulation properties. Therefore, there is a demand for a gas adsorbent that can adsorb gas in a pressure range not higher than atmospheric pressure.
Such gas adsorption is required to be irreversible; therefore, physical adsorption is unsuitable and chemical adsorption, which forms stronger bond, is desirable. Nitrogen, which makes up 80% of the air, however, has a stable triple bond, making the chemical adsorption very difficult.
Examples of an alloy that adsorbs such nitrogen, especially, an alloy that removes such nitrogen at low temperature, include a Ba—Li alloy (for example, see Patent Literature (PTL) 1).
The Ba—Li alloy is used as a device for maintaining vacuum in a thermally insulating jacket together with a drying material and shows high reactivity towards gas such as nitrogen even at room temperature.
Ba used in a conventional adsorbent in PTL 1, however, is a substance designated by the pollutant release and transfer register (PRTR), and therefore, for the purpose of industrial use, it is necessary to make Ba absorbable by a substance that poses no problem for the environment and human bodies. There is also the problem that, in order to expand the application of the vacuum thermal insulation material, the vacuum thermal insulation material is required to be able to adsorb an increased volume of nitrogen.